In contrast to alpha- and beta-particles, neutrons do not have a charge and therefore cannot lose energy by ionization on passing through matter. Consequently, their penetration power is extremely high. Neutrons are influenced exclusively by the action of nuclear forces and are scattered on atomic nuclei. According to the collision laws, the energy released to the body receiving the impact is greater the more similar the mass thereof is to the mass of the colliding body. Therefore, a bundle of neutron beams, which penetrates lead plates of several meters thickness without significant attenuation, is very greatly attenuated when passing through hydrogen-containing substances of a few cm thickness. On average, the energy is reduced to l/e on collision with a proton, whereas the energy release to atomic nuclei of higher mass is less, due to inelastic collision. It is known from the literature that, on average, 18 collisions are necessary in hydrogen and 114 collisions are necessary in carbon in order to brake a neutron down to thermal energy. These thermal, i.e. slow, neutrons can then be completely absorbed by elements of high effective cross-section, such as cadmium or boron.
In neutron absorption, binding energy is released in the form of secondary gamma-radiation. Its nature depends on the absorbant and can be of considerable magnitude. Thus, the gamma-radiation energy is 6 MeV in the absorption of neutrons by cadmium, 2.2 MeV by hydrogen, and only 0.5 MeV by boron. As the materials which protect against neutron radiation, especially water and paraffins as well as plastics containing significant quantities of hydrogne, such as polyethylene, polyesters and polyamides, are used.
Thus, according to the teaching of German Auslegeschrift 1,297,869, moldings of thermoplastic or thermosetting plastics, in which the carbon/hydrogen ratio or the residual atom/hydrogen ratio is in the range of 1:2.1 to 2:1 and the molecular weight of which is less than 200,000, are used for protection against gamma-radiation and neutron radiation. Such plastics can be from the classes of high- and low-pressure polyethylenes, polypropylenes, alkylene/propylene or alkylene/butylene copolymers, polyamides, and polyesters.
In German Auslegeschrift 11 62 694, a neutron-absorbing material is described in which granulated polyethylene is embedded in a hydrogen-containing liquid which remains liquid or cures to give a plastic. However, the known neutron-absorbing materials have properties which restrict their applicability. Thus, although plastics have a low density, their processibility frequently causes difficulties. Moreover, their mechanical behavior does not always meet all requirements and their heat resistances are frequently unsatisfactory.